1. Field of the Invention
The present invention relates to a photothermographic material and an image forming method. More particularly, the invention relates to a double-sided type photothermographic material using a silver halide emulsion with a high silver iodide content and an image forming method utilizing the same. Further, the invention relates to a double-sided type photothermographic material whose photographic property is improved and an image forming method utilizing the same.
2. Description of the Related Art
In recent years, in the medical field and the graphic arts field, there has been a strong desire for a dry photographic process from the viewpoints of environmental conservation and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and output by communicating it to a desired location where the image information is output onto a photosensitive material using a laser image setter or a laser imager, and developed to form an image at the location on the photosensitive material. It is necessary for the photosensitive material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using a pigment or a dye, such as inkjet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording material, images in the digital imaging recording material obtained by such a general image forming system are insufficient in terms of image quality (sharpness, granularity, gradation, and tone) needed for medical images used in making diagnoses and high recording speed (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
A photothermographic material using an organic silver salt has already been known. Generally, the photothermographic material has an image forming layer in which a photosensitive silver halide, a reducing agent a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of silver are dispersed in a binder.
A photothermographic material forms a black silver image by being heated to a high temperature (for example, 80° C. or higher) after image wise exposure to cause an oxidation seduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. There is much literature in which photothermographic materials are described, and the Fuji Medical Dry Imager FM-DP L is a practical example of a medical image forming system using a photothermographic material that has been marketed.
Since the image forming system utilizing an organic silver salt has no fixing step, undeveloped silver halides remain inside the film after thermal development. Thus, there have intrinsically been serious problems in the system.
One of them involves image instorability after a thermal developing process, particularly fogging due to print-out when the material is exposed to light. As a means to improve print-out, a method of using silver iodide is known. Silver iodide has the characteristic of causing less print-out than silver bromide or silver iodobromide having an iodide content of 5 mol % or less, and has a potential for fundamentally solving the problem. However, the sensitivity of silver iodide grains known until now is extremely low, and the silver iodide grains do not achieve a level of sensitivity that is applicable for an actual system. When means of preventing recombination between photoelectrons and holes is performed to improve the sensitivity, it is an inherent problem that the characteristic of being excellent in the print-out property will be lost.
As means of increasing the sensitivity of a silver iodide photographic emulsion, academic literature discloses addition of a halogen acceptor such as sodium nitrite, pyrogallol, hydroquinone or the like, immersion in an aqueous silver nitrate solution, sulfur sensitization at a pAg of 7.5, and the like. However, the sensitization effect of these halogen acceptors is very small and extremely insufficient for use in photothermographic materials.
On the other hand, attempts have also been made at applying the abovementioned photothermographic material as photosensitive material for photographing. The “photosensitive material for photographing” as used herein means a photosensitive material on which images are recorded by a one-shot exposure through a lens, rather than by writing the image information by a scanning exposure with a laser beam or the like. Conventionally, photosensitive materials for photographing are generally known in the field of wet developing photosensitive materials, and include films for medical use such as direct or indirect radiography films, mammography films and the like, various kinds of photomechanical films used in printing, industrial recording films, films for photographing with general-purpose cameras, and the like. For example, an X-ray photothermographic material coated on both sides using a blue fluorescent intensifying screen described in Japanese Patent No. 3229344, a photothermographic material containing tabular silver iodobromide grains described in Japanese Patent Application Laid-Open (JP-A) No. 59-142539, and a photosensitive material for medical use containing tabular grains that have a high content of silver chloride and have (100) major faces, and that are coated on both sides of a support, which is described in JP-A No. 10-282606, are known. However, there have conventionally been no descriptions about a thermal developing apparatus for these double-sided type photothermographic materials.
Photosensitive materials comprising tabular silver iodide grains as silver halide grains are well known in the wet developing field as described in JP-A Nos. 59-11934 and 59-119350, but there have been no examples of the application of the silver iodide grains in a photothermographic material. The reasons for this are because, as mentioned above, the sensitivity is very low, there are no effective sensitization means, and the technical barriers become even higher in thermal development.
In order to be used as this kind of photosensitive material for photographing, the photothermographic material needs higher sensitivity as well as an even higher level of image quality, such as the degree of haze of an obtained image.